Single layer or monowall fuel lines and vapor return lines constructed of synthetic materials such as polyamide have been proposed and employed. Fuel lines employing such materials generally have length of at least several meters. It is important that the line, once installed, not materially change during the length of operation, either by shrinkage or elongation as a result of the stresses to which the line may be subject during use. Heretofore, monowall tubing composed of fluoroplastic materials has not been desirable due to the dimensional instability of the fluoroplastic materials during usage over time.
It is also becoming increasingly important that the fuel and vapor return lines employed be essentially impervious to hydrocarbon emissions due to permeation through the tubing. It is anticipated that future federal and state regulations will fix the limit for permissible hydrocarbon emissions due to permeation through such lines. Regulation which will be enacted in states such as California will fix the total passive hydrocarbon emission for a vehicle at 2 g/m.sup.2 per 24 hour period as calculated by evaporative emission testing methods such as those outlined in Title 13 of the California Code of Regulations, .sctn.1976, proposed amendment of Sep. 26, 1991. In order to achieve the desired total vehicle emission levels, a passive hydrocarbon permeation level for fuel and vapor return lines equal to or below 0.5 g/m.sup.2 per twenty-four hour period would be highly desirable, if not required. Heretofore, it has been difficult to meet the passive hydrocarbon permeation levels outlined with tubing of monowall polyamide construction. However, as indicated previously, these materials fail to provide the dimensional stability and rugged durability characteristics necessary for fuel and vapor return lines. Additionally, while the fluoroplastic polymers provide greater resistance to hydrocarbon permeation, it has not been definitively proven that these materials, when employed in monowall construction, provide sufficient permeation resistance to meet the new stricter standards.
Additionally, any material employed in a fuel or vapor recovery line which contact the organic material transported therein must be impervious to interaction with the fuel and with materials present therein such as oxidative agents and surfactants as well as fuel additives such as methanol and ethanol.
In the past, various multiple layer tubing has been proposed in an attempt to address this final concern as well as those previously discussed. In general, the most successful of these have been multi-layer tubing which employ a relatively thick outer layer composed of a material resistant to hazards found in the exterior environment, i.e. UV degradation, road hazard, extreme of heating and cooling, corrosive agents such as zinc chloride and the like. The innermost, fuel contacting, layer is thinner and is generally composed of a material chosen for its ability to block diffusion of materials contained in the fuel to the thick outer layer. Materials for which diffusion is blocked include aliphatic hydrocarbons, alcohols and other materials generally present in fuel blends. The material of choice for the inner layer generally has been a polyamide such as Nylon 6, Nylon 6.6, Nylon 11 and Nylon 12.
Alcohol and aromatic compounds diffuse through the tubing wall at different rates from aliphatic compounds contained in the fuel. This diffusion differential results in a change in the composition of the liquid contained in the fuel or vapor recovery tube which, in turn, can change the solubility threshold of the tubing material. The change in the solubility threshold of the tubing material can result in the crystallization of monomers and oligomers of materials such as Nylon 11 and Nylon 12 into the fuel liquid. The presence of copper ions in the fuel which are introduced from copper present in the fuel pump, can accelerate this crystallization. The crystallized precipitant can block filters and fuel injectors and collect to limit the travel of mechanisms such as the fuel pump or carburetor float. Additionally, crystallized precipitant can build up on critical control surfaces of the fuel pump.
Various tubing materials have been suggested to address these concerns. In U.S. Pat. No. 5,076,329 to Brunnhofer, a five-layer fuel line is proposed which is composed of a thick corrosion-resistant outer layer formed of a material which is durable and resistant to environmental degradation such as Nylon 11 or Nylon 12. Intermediate to this outer layer and protected from environmental degradation is a thick intermediate layer composed of conventional Nylon 6. The outermost and intermediate layers are bonded together by a thin intermediate bonding layer composed of an ethylene or a polypropylene having active side chains of maleic acid anhydride. A thin innermost fuel contacting layer of after condensed Nylon 6 with a low monomer content is bonded to the intermediate Nylon 6 layer by a solvent blocking layer formed of a co-polymer of ethylene and vinyl alcohol having an ethylene content between about 30% and about 45% by weight. In order to achieve bonding between the different polyamides, the specific bonding layers are employed. The use of the five-layer system was mandated in order to obtain a tubing with the exterior resistance characteristics of Nylon 12 and the low monomer/oligomer production characteristic of Nylon 6.
In U.S. Pat. No. 5,028,833 also to Brunnhofer, a three-layer fuel line is proposed in which a multi-layer tube is formed having a co-extruded outer exterior environmental contacting wall of Nylon 11 or Nylon 12, an inner water-blocking wall of Nylon 11 or Nylon 12 and an intermediate alcohol barrier and bonding layer formed of an ethylene-vinyl alcohol copolymer. In DE 40 06 870, a fuel line is proposed in which an intermediate solvent barrier layer is formed of unmodified Nylon 6.6 either separately or in combination of blends of polyamide elastomers. The internal layer is also composed of polyamides preferably modified or unmodified Nylon 6. The outer layer is composed of either Nylon 6 or Nylon 12.
Various devices have also been suggested in which a fluoropolymer is employed as an interior layer in a multi-layered tubing. In EP 0551094 A1 to Krause, a two or three layer fuel line is proposed in which the innermost layer is a fluoropolymer. The fluoropolymer of choice is, preferably, an ethylene tetrafluoroethylene co-polymer extruded and rendered surface active by plasma discharge or corona discharge. A top layer composed of a thermoplastic resinous material such as Nylon 12 is extruded onto the previously formed inner layer. In U.S. Pat. No. 5,170,011 to Martucci, a lightweight hose assembly is suggested in which the inner layer is a polymeric fluorocarbon while the outer layer is an expanded polyamide material. In order to achieve an appropriate permanent bond between the two layers, an intermediate braiding or, alternately, an abraded surface preparation must be prepared in the exterior face of the fluoropolymer layer so that a mechanical bond can be obtained with the polyamide material. Heretofore, it has been impossible to provide a dual or multiple layer tubing with satisfactory lamination characteristics between the dissimilar polymer layers.
Thus, it would be desirable to provide a tubing material which could be employed in motor vehicles which would be durable and have an exterior oriented surface which is resistant to environmental degradation and durable through prolonged use. It would also be desirable to provide a tubing which would reduce permeation of organic materials therethrough. It would also be desirable to provide a tubing material which would be essentially non-reactive with components of the fuel being conveyed therein. It is also an object of the present invention to provide a method for preparing a multi-layer tubing of the present invention by coextrusion.